Electromagnetic Spectrum– Regions of interest to the Chemical Spectroscopist

Electromagnetic Spectrum– Regions of interest to the Chemical Spectroscopist

Electromagnetic radiation is radiant energy that is transmitted through space at the speed of light. Humans can experience it in the form of light and heat over a very small fraction of its almost infinite range of its wavelengths. It is true that electromagnetic radiation does not require any medium for progression but it can be felt as heat or light only when it passes through a media like air.

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Visible region-a small part of complete spectrum (Image Courtesy : http://www.pion.cz/)

Interaction of electromagnetic radiation with matter is of key interest to the chemist. Spectroscopy is a science that is based on this interaction and provides valuable information on the characteristics of materials, their identification and quantification. The information provided in different regions relates to different types of energy interactions at the molecular or atomic levels.

Expression of electromagnetic waves

Electromagnetic radiation can be visualized as oscillating magnetic and electric fields propagating in planes perpendicular to each other and to the direction of propagation.

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Components of electromagnetic wave (Image Courtesy : http://www.ualberta.ca/)

The radiation can be characterized in terms of several parameters such as:

Amplitude – refers to the amplitude of the crests or troughs of the electromagnetic wave.

Wavelength – the linear distance between two consecutive crests or troughs that constitute the complete cycle. It is expressed as lambda(λ) in terms of units of distance and varies from angstroms to several meters.

Frequency – the number of crests or troughs that pass through a given point every second and is expressed in terms of cycles per second or Hertz. The relationship between velocity, frequency and wavelength is

v= c/λ

where λ is in meters, v is in \(s^-^1\)and c is \(3X 10^8 m s^-^1\)

Wave number – another expression for wavelength which defines the  waves per centimetre and is denoted as a number\((\overline{v})\) where

\(\overline{v}\)=1/λ(cms)

Velocity – the linear distance travelled by the wave in one second. It can be expressed as

C=vλ

Energy   of the radiation depends on both the wavelength and frequency

E=hv=hc/λ

where h is Planck’s constant and has a value of 6.626 \(X10^-^2^4\) Joules.sec.

Wavelengths in different spectral regions

The Electromagnetic spectrum comprises of a vast expanse in terms of wavelength and energy. The chemical spectroscopist has exploited regions ranging from gamma rays to microwaves for studies on different materials though the spectrum extends to even higher wavelength regions like radio waves. For convenience sake the wavelengths are expressed in units which are multiples of meter which is the universal unit of length in the SI system of measurements. The table below summarizes the spectroscopic regions along with the units of wavelength used to express the wavelength ranges.

Region of EM SpectrumWavelength RangeMolecular or Atomic Interactions
Gamma Rays0.01 – 0.1 AngstromsEjection of core electrons in heavy metals
X-Rays0.1 – 10 nmEjection of core electrons
UV10-200 nmExcitation of valence electrons
Visible200 – 800 nmMolecular excitation
Infra Red(Near to Far IR)2500 – \(10cm^-^1\)Molecular Vibrations and rotations
Microwaves0.01 – 10 cms Molecular rotations

The boundaries of the regions are not well defined and there is often overlap to some extent. The light sources are often broad band sources and narrow bands required for spectroscopic studies are isolated using monochromators.

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